1. Field of the Invention
The present invention relates to a detoxifying agent that is used when noxious wastes are treated with cement to solidify them. More particularly, the present invention relates to a detoxifying agent for use in disposal of noxious wastes, capable of providing cement-solidified materials having high strength, substantially free from exudation of toxic substances such as heavy metals, PCBs and dioxins.
2. Background Art
How to finally dispose of noxious wastes such as residues from the incineration of refuse, component (especially, molten slag and heavy metals) from the incineration of hazardous industrial wastes, polluted soil, river sludge, sewage sludge, waste glass and waste tires is now a serious social problem. These noxious wastes can discharge a variety of toxic substances, and, of these toxic substances, heavy metals, PCBs, dioxins, etc., in particular, are known to exert serious effects on ecosystem. For this reason, there has been strongly demanded a technique of finally disposing of noxious wastes without discharging the above toxic substances.
To meet the above demand, there have been proposed various methods for making noxious wastes innoxious, and subjecting them to recycling. One of these methods proposed is as follows: noxious waste containing the above-described toxic substances is mixed with cement, and the mixture is solidified to enclose the toxic substances in the cement; the cement-solidified material thus obtained is then subjected to recycling. However, the toxic substances tend to inhibit the agglomeration of cements, so that they retard the hardening of cement, and impart decreased strength to the cement-solidified material. Another problem is that the toxic substances enclosed in cement can ooze out of the cement-solidified material.
To solve these problems, the following method has been proposed as disclosed in Japanese Patent Laid-Open Publication No. 24276/1998: a solution containing, as main components, tannin and wood vinegar is diluted with water, and this aqueous solution is added as a detoxifying agent to noxious waste containing such toxic substances as heavy metals and PCBs to detoxify them. It is considered that, since tannin and wood vinegar exert a powerful chelating action on elemental compounds such as heavy metals, the detoxification of heavy metals and the like can successfully be attained by this method. However, although cement-solidified materials obtained by this method show strength to some degree, a further improvement in the strength has been expected.
An object of the present invention is to provide a detoxifying agent that is used when noxious wastes are treated with cement to solidify them, and that can provide cement-solidified materials having high strength, substantially free from exudation of toxic substances such as heavy metals, PCBS and dioxins.
The present invention provides a detoxifying agent for use in disposal of noxious wastes that is conducted by solidifying them with cement, consisting essentially of an aqueous solution which comprises as main components (a) small-sized metal cations whose 6-coordination ion radii are less than 100 pm, (b) medium-sized metal cations whose 6-coordination ionic radii are between 100 pm and 140 pm, and (c) large-sized metal cations whose 6-coordination ionic radii are in excess of 140 pm. The present invention also provides cement-solidified materials that can be obtained through the use of the above-described detoxifying agent.
The detoxifying agent according to the present invention, for use in disposal of noxious wastes that is conducted by solidifying them with cement will be described hereinafter.
In the present invention, the xe2x80x9cdetoxifying agentxe2x80x9d is an additive that is used when noxious wastes are treated with cement to solidify them. The purposes of this agent are to detoxify toxic substances contained in noxious wastes, thereby enclosing the detoxified substances in cement without discharging them, and to prevent toxic substances from exerting their effect of inhibiting the setting of cement, thereby obtaining cement-solidified materials having increased strength.
In the present invention, the xe2x80x9ctreatment with cement for solidificationxe2x80x9d is conducted for the purpose of disposing of noxious wastes. This treatment is useful in enclosing, in cement, such toxic substances as heavy metals, PCBs and dioxins contained in noxious wastes, thereby obtaining recyclable cement-solidified materials.
The xe2x80x9cnoxious wastesxe2x80x9d that can be disposed of by the present invention include a variety of wastes that can discharge toxic substances such as heavy metals, dioxins and PCBs, and there is no particular limitation on them. Examples of such noxious wastes include residues from the incineration of refuse, component (especially, molten slag and heavy metals) from the incineration of hazardous industrial wastes, polluted soil, river sludge, and sewage sludge. In addition, waste glass, tires, plastics and CD, and the like are also included in the noxious wastes.
As mentioned previously, even if the above-described noxious wastes are treated with cement and solidified, toxic substances enclosed in the cement can ooze out of the cement-solidified materials. Moreover, since the toxic substances have the effect of inhibiting the setting of cement, the hardening of cement is retarded, and the resulting cement-solidified materials are to have decreased strength.
We found that it is possible to make the degree of exudation of toxic substances extremely low, and, at the same time, to obtain cement-solidified materials having increased strength if a detoxifying agent having a specific composition is used when noxious wastes are treated with cement to solidify them.
A detoxifying agent according to the present invention consists essentially of an aqueous solution comprising as main components (a) small-sized metal cations whose 6-coordination ionic radii are less than 100 pm, (b) medium-sized metal cations whose 6-coordination ionic radii are from 100 to 140 pm, and (c) large-sized metal cations whose 6-coordination ionic radii are more than 140 pm.
The xe2x80x9c6-coordination ionic radiixe2x80x9d herein mean ionic radii where the coordination number is 6, and refer to the generally known values that were determined by Shannon and Prewitt on the basis of observed values, and improved by Shannon. These ion radii (CR: crystal ionic radii) are enumerated on pages 725-726 of xe2x80x9cKaitei Yon-han, Kagaku Binran, Kiso-hen II (Handbook of Chemistry, Basic Section II, Fourth Revised Edition)xe2x80x9d, edited by The Chemical Society of Japan, published by Maruzen Co., Ltd., Japan.
The 6-coordination ionic radii of major ions useful for the present invention are as follows:
(a) as small-sized metal cations: Li+: 90 pm, Mg2+: 86 pm, Mn2+: 81 pm, 97 pm, Fe3+: 69 pm, 79 pm, Ni2+: 83 pm;
(b) as medium-sized metal cations: Na+: 116 pm, Ca2+: 114 pm, Sr2+: 132 pm; and
(c) as large-sized metal cations: Ba2+: 149 pm, K+: 152 pm, Rb+: 166 pm.
In addition, examples of halide anions include Clxe2x88x92 and Brxe2x88x92, and their ionic radii are 167 pm and 182 pm, respectively. Regarding the above-described ionic radii of Mn2+ and Fe3+, the former value shows the radius of the ion that is in the low spin state, and the latter one shows the radius of the ion that is in the high spin state.
It is considered that, since metal cations selected from each of the three groups of ions that are different in the range of ionic radii are incorporated into the detoxifying agent of the present invention, the detoxifying agent can exert various detoxifying actions, such as a coagulating effect, on a wide variety of toxic substances. In the solidification of noxious wastes with cement, various toxic substances are considered to exist, and even if only a few of these toxic substances have the effect of inhibiting the setting of cement, it is unavoidable that the hardening of cement is adversely effected. Therefore, to incorporate, into the detoxifying agent, metal cations selected from each of the above-described groups (a), (b) and (c), which are different in the range of ionic radii, is considered to be quite effective in making the detoxifying agent useful for the detoxification of a wide variety of toxic substances.
(a) Small-Sized Metal Cations
Small-sized metal cations for use in the present invention are metal cations whose 6-coordination ionic radii are less than 100 pm. Preferably, the small-sized metal cations include at least one ion selected from alkali metal ions, alkaline-earth metal ions and 3d transition metal ions. More preferably, the metal cations include ions selected from each one of these three groups. It is considered that, since two or more different types of ions are thus allowed to exist in the detoxifying agent, a wider variety of toxic substances can be detoxified. The xe2x80x9c3d transition metalsxe2x80x9d herein refer to the transition metals with atomic numbers of 21 (Sc) to 29 (Cu), having an unfilled 3d shell; this transition metal series is also called the elements of the first transition series.
There is no particular limitation on the small-sized metal cations for use in the present invention. Preferable examples of the metal cations include Li+, Mg2+, Mn2+, Fe3+ and Ni2+, and it is most preferable to use all of these ions as the small-sized metal cations. It is preferable that these metal cations be supplied to the aqueous solution in the form of halides. It is more preferable that the metal cations such be supplied to the aqueous solution in the form of, for instance, lithium chloride, magnesiumchloride, manganese (II) chloride, iron (III) chloride, and nickel (II) chloride. This is because, since these halides are strong electrolytes, they are completely ionized in the solution.
(b) Medium-Sized Metal Cations
Metal cations of any type can be used as the medium-sized metal cations for use in the present invention as long as their ionic radii fall in the range of 100 to 140 pm. The medium-sized metal cations preferably include at least one ion selected from alkali metal ions and alkaline-earth metal ions, more preferably ions selected from both of these two groups. It is considered that, since two or more types of ions are thus allowed to exist in the detoxifying agent, the detoxification of a wider variety of toxic substances can successfully be attained.
Specific examples of the medium-sized metal cations that can preferably be used in the present invention include Na+, Ca2+ and Sr2+, and it is most preferable to use all of these ions. It is preferable that these metal cations such be supplied to the aqueous solution in the form of halides. It is more preferable that the metal cations such be supplied to the aqueous solution in the form of, for example, sodium chloride, calcium chloride and strontium chloride. This is because, since these halides are strong electrolytes, they are completely ionized in the solution.
(c) Large-Sized Metal Cations
The large-sized metal cations for use in the present invention are those ones whose ionic radii are greater than 140 pm. Preferably, the large-sized metal cations include at least one ion selected from alkali metal ions and alkaline-earth metal ions. More preferably, the metal cations include ions selected from both of these two groups. It is considered that, since two or more types of ions are thus allowed to exist in the detoxifying agent, a wider variety of toxic substances can successfully be detoxified.
Specific examples of the large-sized metal cations that can preferably be used in the present invention include Ba2+, K+ and Rb+, and it is most preferable to use all of these ions. It is preferable that these metal cations such be supplied to the aqueous solution in the form of halides. It is more preferable that the metal cations in the form of, for example, barium bromide, potassium chloride and rubidium chloride be supplied to the aqueous solution. This is because, since these halides are strong electrolytes, they are completely ionized in the solution.
In the detoxifying agent of the present invention, it is preferable to use, as the metal cations, alkali metal ions, alkaline-earth metal ions and 3d transition metal ions as described hereinbefore. The reason why ions of these three types are used is that these ions are geochemically important and stable in any environment over a long period of time. Alkali metal ions have strong salting-out power (show high bond strength with water), and tend to coagulate colloidal toxic substances, so that they are considered to act to detoxify toxic substances. Alkaline-earth metal ions form sparingly soluble salts, and have solidifying, hardening and adhering effects. They are therefore considered to act to detoxify toxic substances. 3d Transition metal ions form sparingly soluble salts or complexing salts, so that they are considered to have the effect of detoxifying toxic substances.
The detoxifying agent of the present invention thus comprises a variety of metal cations including ions whose ionic radii fall in at least three different ranges, the ions being preferably selected from each one of the three groups of alkali metal ions, alkaline-earth metal ions and 3d transition metal ions. The detoxifying agent of the invention can therefore harmoniously exert, on a variety of toxic substances, such effects as coagulating, dehydrating and complexing effects, and the effect of forming sparingly soluble compounds (precipitates). It is considered that, since these effects are synergistically exerted on various toxic substances, the toxic substances are detoxified more efficiently and completely.
In the present invention, various optional components may be incorporated into the detoxifying agent as needed within the scope of the invention.
It is noted that, since such components as tannin and wood vinegar can inhibit the hardening of cement as can be known from the results obtained in Example 2 and Comparative Example 2, it is desirable to incorporate substantially no tannin and wood vinegar into the detoxifying agent of the present invention.
There is no particular limitation on the condensation of each of the above-described metal ions in the detoxifying agent of the present invention, or on the ionic strength of the detoxifying agent. These two properties may be properly decided depending on the waste to be disposed of, or on the manner in which the detoxifying agent is used.
According to a preferred embodiment of the present invention, the detoxifying agent is prepared so that it will have an ionic strength of preferably 1 or more, more preferably 2 to 4. The detoxifying agent can more efficiently exert various effects such as a coagulating effect on toxic substances as long as it has an ionic strength in the above range.
According to a preferred embodiment of the present invention, the detoxifying agent is prepared so that its pH will fall in the range of 6 to 8. The detoxifying agent of the present invention having such a pH is advantageous in that it is nearly harmless to human body, is easy for handling and preservation, and can be a stable aqueous solution.
The detoxifying agent of the present invention may be prepared by a method that is selected properly depending upon the types of components to be used, and there is no particular limitation on the production method. For example, in the case where alkali metal ions, alkaline-earth metal ions and 3d transition metal ions are used, the following method may be employed. Halides of alkali metals are firstly placed in a first vessel, while halides of alkaline-earth metals and those of 3d transition metals are placed in a second vessel. Hot water at about 90xc2x0 C. or higher is poured into these vessels so that the ratio by volume of the quantity of the hot water in the first vessel to that of the hot water in the second vessel will be from 6:4 to 8:2, followed by stirring. The aqueous solution in the first vessel and that in the second vessel are then mixed. This mixture is stirred again, and cooled to room temperature to give a detoxifying agent of the present invention.
The detoxifying agent of the present invention is mixed with noxious waste to be disposed of, water and cement, and there is no particular limitation on the order of mixing.
According to a preferred embodiment of the present invention, the detoxifying agent is used in the following manner. After adding the detoxifying agent to noxious waste and agitating the mixture, cement is added to this mixture, and the resultant mixture is further agitated; a proper amount of water is then added to this mixture, and the mixture obtained is thoroughly agitated. When the detoxifying agent is used in this manner, the effects of the present invention can be obtained more efficiently.
Any type of cement including Portland cement, Portland blast furnace slag cement and fly ash cement can be used together with the detoxifying agent of the present invention. It is however preferable to use ordinary Portland cement since this cement shows sufficiently high initial strength, is economically advantageous, and is effective in obtaining cement-solidified materials that are stable in quality.
The detoxifying agent of the present invention is considered to function in the following way. The detoxifying agent exhibits its various effects as the hydration reaction of cement proceeds, and needle crystals are thus produced in pores of various sizes formed in the hardened cement. These needle crystals are crystals having no water unlike simple hydrates of cement, so that the hardened cement is to have a denser constitution. It is considered that, owing to this denser constitution, the hardened cement shows increased strength and durability, and becomes slow to undergo corrosion.
The cement-solidified material obtained through the use of the detoxifying agent of the present invention is such that toxic substances contained in noxious waste have efficiently been detoxified due to the various effects of the detoxifying agent, such as the effect of forming sparingly soluble compounds (precipitates), and coagulating, hydrating and complexing effects. Therefore, the cement-solidified material exudes substantially no toxic substances, and is excellent in both strength and durability.
It is preferable to reuse the cement-solidified materials as durable materials. Preferable examples of durable materials include secondary products of concrete such as interlocks, flat plates, concrete curbs, car stops, and tetrapod. Thus, it becomes possible to recycle noxious wastes efficiently and safely.